Conventional contact lenses are generally designed to improve the visual acuity of the human eye. The human eye frequently does not constitute a perfect optical system, but instead comprises optical imaging errors which are known as ocular aberrations. For many individuals, the most significant refractive errors giving rise to visual problems associated with the focal power of the eye are those caused by the so-called lowest order aberrations, namely defocus and astigmatism. However, a large number of people have normal retinal function and clear ocular media, but cannot attain good visual acuity using conventional contact lenses, due to having an irregular corneal surface or irregular crystalline lens which gives rise to higher order or irregular aberrations.
One such defect of the eye is known as spherical aberration, wherein light rays entering a refracting surface such as the cornea of the eye, or the lens, are less strongly focused at the centre of the cornea or lens and are progressively more strongly focused off-centre. This results in an image that is sub-optimal and is not clearly focused on one point, but on a series of focal points in front or behind the retina. UK Patent No. 2364138 relates to the design of contact lenses for reduction of spherical aberration. Lenses have also been developed to reduce the effect of tilt aberrations.
Several techniques have been used to measure the wavefront aberrations of the eye. One such technique involves the measurement of the ocular shape of a wavefront using a wavefront aberrometer. The wavefront aberrometer operates by focusing a fine beam of light through the eye. The beam is then scattered back by the retina, through the eye and the pupil. The resulting data is presented as a profile of the optical aberrations present in a subject's eye by means of an ocular optical wavefront. In this respect, the aberrations of the eye can be described as a defined wavefront, the shape of which represents the optical aberrations, a planar wavefront being produced by an ocular system with no net aberrations. The wavefront may be mathematically described using a number of functions, most commonly using a set of Zernike or Seidal functions.
The surface topography of the cornea of the eye is often not spherical. Instead the corneal topography may change at varying and unpredictable rates from the centre of the cornea out to the periphery in a manner that is unique to every individual cornea. Consequently, the optical function of a contact lens is affected by the topography of the cornea onto which the contact lens is superposed. In some cases, the uneven topography the cornea may affect the contact lens by creating a layer of lachrymal fluid (i.e. a lachrymal lens) between the posterior face of the contact lens and the surface of the cornea and/or by deforming the posterior (and consequently the anterior surface in the case of a soft lens) of the contact lens when the contact lens is superposed on the cornea of the eye.
U.S. Pat. No. 5,114,628 describes a method for the manufacture of contact lenses whilst taking into account the topography of the eye and compensating for the lachrymal lens by modifying the optical area of the front surface of the lens. The document does not disclose correction of higher order aberrations.
U.S. Pat. No. 6,086,204 describes a method to record the aberrations of the eye. In particular, the method involves the use of mathematical steps to design a contact lens wherein the aberrations have been significantly reduced. However, there is no reference in this patent to the feature wherein a soft contact lens is designed to take into account the topography of the cornea. The method disclosed in U.S. Pat. No. 6,086,204 requires the actual manufacture of an initial lens, with associated cost and effort of fitting, etc.
U.S. Pat. No. 6,305,802 describes a method of integrating the topography of the cornea and ocular wavefront data to create a soft lens design. The method involves the use of topographical data to create a soft contact lens design wherein the posterior surface of the lens is uniquely matched to the corneal topography. In addition, the unique posterior surface of the lens corrects for lower and higher order optical aberrations. The posterior surface of the contact lens is described as having an arbitrarily defined shape or an averaged shape based on the corneal topography, wherein any known aberrations are corrected on the anterior surface of the lens. This involves a process of using corneal topographic information to calculate the posterior surface of the lens, subtracting optical aberrations due to the posterior surface of the lens from the total optical aberrations, and using the net residual optical aberration to determine the anterior surface of the lens. However, there is no reference or guidance given as to the method of calculating the resulting ocular aberrations using the configuration of the posterior surface of the lens.
International Application No. PCT/US2003/027478 describes a method of using a real trial diagnostic lens in order to determine the ocular aberrations that arise from the combination of a soft contact lens with an arbitrarily defined posterior surface. The trial lens is also used to determine the position of the final lens in normal wear conditions. However, the use of such a trial lens increases the cost and time required for fitting contact lenses.